1. Technical Field
Physical properties of materials (e.g., resistance to galling, coefficient of friction, and wear rate) as they are moved relative to each other while in contact are measured.
2. Related Art
Wear-resistant couples are used in literally thousands of applications in which the couples slide while in contact with each other. The vast majority of such wear-resistant couples are made from metals. However, some couples are made from ceramics or from mixtures that include some ceramics and other solid materials.
In the process of designing mechanical equipment made from materials such as metals and ceramics, it is important to have knowledge of suitable pairs of materials or material coatings that must move in contact with each other at a high level of contact pressure so that undesirable events such as galling may be prevented. Galling of metals, also referred to as “cold welding”, is said to occur when one surface in contact adheres to the other surface with such strong adhesion that a part of one of the surfaces is torn away, and both surfaces become damaged. This phenomenon occurs at temperatures far below softening or melting point of metals, hence the term “cold welding”.
Galling can occur between metal surfaces that are dry and unlubricated, or lubricated, or wetted by some fluid. Different pairs of metals moving in contact under a high level of stress have different resistance to galling, and the designer must select a metal pair that has a high enough degree of galling resistance, hereinafter referred to as “GR”, to prevent damage to surfaces of the coupling parts.
A non-exhaustive list of areas in which knowledge of GR is important include:                Sleeve-type bearings, where shafts rotate in supporting sleeves, sometimes with considerable forces pressing the parts together;        Internal parts for valves, where the parts are forced together by high pressure gas or steam or liquid that the valves are containing, as they slide against each other while the valves are operated; and        Material selection for nuts and bolts and machine screws, where there is sliding of contact surfaces of threads as the nuts, bolts, and/or screws are tightened, usually with high contact pressure.        
There are thousands of similar industrial applications where GR between metal pairs is important to know, to assure reliable product designs. Knowledge of GR for couples is important to enable sound design.
It is also significant to have information on coefficient of friction, hereinafter referred to as “COF”, of couples, so that inherent resistance of the material pair to slide, one on the other, is known. This knowledge allows a design in which adequate force is supplied to produce sliding motion required in the equipment under design. The COF can be defined as the force required to produce sliding between two parts held together in contact, divided by the force that is holding them together. Less force is required to produce sliding when COF is low. Thus, in some instances, selection of metal couples should be considered for adequate GR, and also for lowest, or at least adequately low, COF.
Considering the importance of having these types of knowledge, there is surprisingly limited information available in the technical literature on GR and COF between various load-bearing metal couples. This is true even for those that have been in common use for many years. In addition, new metal alloys, new galling resistant coatings, and coating methods are introduced every year. There is much information on their strength and hardness and bond strength. However, there is an almost complete lack of data on their GR or COF, when used in contact with themselves or in combination with large number of other alternatives. Examples include new metal carbide and metal nitride coatings that can be extremely useful in preventing wear and galling and in lowering friction between parts, but virtually no data is available for their threshold GR, i.e., the contact pressure at which galling commences for that particular couple, nor for their COF. There is also similar lack of information on wear rates, hereinafter referred to as “WR”, of couples.